The present disclosure relates to industrial shaft drives and particularly shaft drives for grinding mills employing relatively large drive motors in the range of 4,000 to 15,000HP and capable of sustaining drive torque in excess of 1,000,000 foot pounds (1,350,000 newton-meters). Typically the drive for such a device comprises a variable speed AC or DC motor driving a grinding mill through a frictional coupling. Heretofore, such frictional couplings have employed pneumatically operated movable members on the motor output shaft to frictionally engage an annular driven surface on the grinding mill power input shaft. Couplings of this type are known to employ a plurality of circumferentially disposed radially movable arcuate elements having friction pads thereon and operable upon pneumatic inflation of an inflatable ring to cause the friction pads on the radially movable elements to engage the annular driven surface and connect the motor shaft to the input shaft of the grinding mill.
In the aforesaid types of pneumatically operated drive coupling for a grinding mill, the radially movable arcuate elements employ an axial passage therethrough which has received therethrough a torque bar having its ends anchored in annular plates attached to the motor power output shaft for rotation therewith. The axial passages through the radially movable arcuate element generally have a rectangular cross section which is elongated in the radial direction preventing movement in the circumferential direction but permit movement in the radial direction of the arcuate elements. Additional axial passages are provided to the arcuate elements to provide for air circulation and cooling. However, in service, structural failure of the radially extending wall portion of the axial passage for the torque bar through the arcuate element has experienced structural failure upon sudden torque increases in the coupling due to abnormal conditions in the grinding mill. Furthermore, upon the occurrence of such abnormal conditions in the grinding mill and sudden increases in the torque required to operate the mill, the addition torque loading has resulted in failures of the drive motor.
Thus, it has been desired to provide a way or means of protecting a drive motor for an industrial grinding mill from failure due to sudden torque increases; and, also to prevent structural failure of the arcuate elements in the pneumatically operated friction coupling between the motor and the grinding mill. Furthermore, the design of the elements to increase their resistance to structural failure would also be prohibitively costly. Additionally, adding structural material to the radially movable arcuate elements to provide the desired increased structural strength would add mass to the rotating elements and increase rotary inertial forces.